In the casings of turbo-engines and particularly multistage turbo-engines, the adapting of the thermal behavior with the passage of time to that of the rotor so as to maintain the radial gap over rotor blades and stator vanes constant upon a change in load is always a problem. It is known that, in particular, the gap around the rotor blades has a great effect on efficiency, compressor stall and fuel consumption. A decrease in the consumption of fuel for the same power and outside dimensions of the power plant is desired.
In the design of this casing the following criteria, among others, are to be taken into account, in addition to the above-mentioned behavior:
a) low weight
b) simple manufacture: large tolerances in the case of inaccessibility/small tolerances in the case of accessibility
c) narrow receiving grooves for the feet of the statorvanes
d) easy assembly
e) easy disassembly
f) rotor bladed and screwed with d) and e)
g) provide grazing layers (narrow blade clearance)
h) easy repair
i) roundness of the casing during manufacture
j) continued roundness of the casing in operation
k) trueness to shape of the casing in axial direction during operation.
The designs known today are generally subdivided into horizontally (or vertically) divided casings (for instance, British Pat. No. 960 812 and Federal Republic of Germany Pat. No. 33 15 914) and casings consisting of rings (Federal Republic of Germany Pat. No. 30 18 621). A cup-shaped casing with suspended segments is another known possibility (for instance, Federal Republic of Germany Pat. No. 33 33 436).
Gap control is generally effected (if provided at all) by blowing against the casing (Federal Republic of Germany Unexamined Application 0S 29 22 835), aeration of the rotor (Federal Republic of Germany Unexamined Application 0S 33 08 140) or the application of weights (Federal Republic of Germany Application 0S 29 07 748) in the casing above the rotor blades.